1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and, more particularly, to a method of fabricating a semiconductor device having an Au electrode, which is adapted to a silicon IC, a transistor, a diode and the like.
2. Description of the Related Art
A semiconductor device having an Au electrode generally has a barrier metal layer formed between the Au electrode and a semiconductor layer to prevent the diffusion of Au. The technique for dry etching of a barrier metal layer essentially with a chlorine based gas in the method of fabricating a semiconductor device having an Au electrode is well known (see Japanese Unexamined Patent Publication No. Sho 62-92323).
FIGS. 1A through 1C present step-by-step cross-sectional views showing a conventional method of fabricating a semiconductor device having an Au electrode. This fabrication method employs the dry etching method disclosed in Japanese Unexamined Patent Publication No. Sho 62-92323 and is adapted to a bipolar transistor as a semiconductor device.
As shown in FIG. 1A, a silicon substrate 1 is prepared. First, a base region 2 is formed in a part of the surface of the silicon substrate 1. A silicon oxide film 8 is formed on the top surface of the silicon substrate 1, and a silicon nitride film 9 is formed on the top surface of the silicon oxide film 8. Then, the silicon oxide film 8 and the silicon nitride film 9 are selectively etched out to form an emitter contact hole 5a. Then, a polysilicon film 4 is so formed as to bury the contact hole 5a, and is subjected to a heat treatment to diffuse the impurity contained in the polysilicon film 4 in the base region 2, thus forming an emitter region 5. Next, a base contact hole 2a is formed in the same manner as the emitter contact hole 5a.
Then, a barrier metal layer 6 is formed by sputtering on the entire top surface of the silicon substrate 1 where the silicon oxide film 8, the silicon nitride film 9 and the like are selectively formed, as shown in FIG. 1B. This barrier metal layer 6 may be formed of a laminated layer, such as a Ti layer-Pt layer or a Ti layer-TiN layer-Pt layer, or a TiW layer or the like. Then, an Au layer is selectively grown by electrolytic plating on the top surface of the barrier metal 6 layer formed in the base contact hole 2a and on the polysilicon film 4, forming an Au electrode 7. With the Au electrode 7 used as a mask, the barrier metal layer 6 is selectively etched off by RIE (Reactive Ion Etching) using, for example, C.sub.2 Cl.sub.2 F.sub.4 as a chlorine based gas. It is taught in Japanese Unexamined Patent Publication No. Sho 62-92323 that O.sub.2 may be added to this etching gas.
Consequently, a semiconductor device having an Au electrode is fabricated as shown in FIG. 1C.
A method of fabricating a beam lead type semiconductor device having an Au electrode is disclosed in, for example, Japanese Unexamined Patent Publication No. Hei 2-84734. This beam lead type semiconductor device is fabricated as follows.
First, a Ti--Pt layer is formed on a semiconductor substrate by vapor deposition. Next, a first Au layer to be an electrode is selectively formed by a photoresist method and plating method. Then, a second Au layer which becomes a beam lead is formed by a photoresist method and plating method with the Ti--Pt layer and first Au layer as a conductive path. With the second Au layer used as a mask, the Ti--Pt layer is dry-etched using a chlorine based gas (e.g., CCl.sub.2 F.sub.2 gas) to which Ar or O.sub.2 gas has been added. The back of the substrate is then processed by polishing and a wet resist method.
According to this prior art, the Ar gas is added to the CCl.sub.2 F.sub.2 gas as the etching gas to reduce the etching rate of Au. To lower the etching rate of Ti as a base film, an O.sub.2 gas is added to the CCl.sub.2 F.sub.2 gas. It is therefore possible to etch the Pt film with a high precision.
Those conventional techniques use a chlorine based gas as the etching gas. Particularly, the use of the CCl.sub.2 F.sub.2 gas or C.sub.2 Cl.sub.2 F.sub.4 gas for dry etching allows the etching selection ratio of Au to the barrier metal and the etching selection ratio of the barrier metal to the base film to be set relatively larger. Further, the amount of re-adhesion (the amount of deposition) of the etched material on the substrate is small so that the wiring material can be etched with a high precision.
Because CCl.sub.2 F.sub.2 and C.sub.2 Cl.sub.2 F.sub.4 belong to specified flons, however, they will be discussed to abolish for the future environmental protection. There is therefore a demand for the development of an etching gas which will replace the etching gases using those specified flons.